Nigeria, May 31 -- Little Karima held her father, Muhammad Nasiru's arm, struggling to keep pace with him. Her flowing gown obscures her uneven gait -the way she swings one leg and limps with the other.

The father raised her gown as they walked some more, exposing her dusty legs from knee to ankle. One of Karima's legs is stiff and bent.

Until mid-last year, Karima's legs were straight, and she already walked well at one year and six months old. But her gait began to change. One of her legs had become stiff, and the little girl was limping.

Karima's result came back positive for circulating Vaccine-Derived Poliovirus type 2 (cVDPV2), a strain of the Wild Polio Virus (WPV) currently endemic in Nigeria. The cVDPV2 is found among populati...